Peripheral artery disease (PAD) and related coronary heart disease (CHD) or cardiovascular disease (CVD) are potential killers.
In the US, an estimated 10 million people have PAD, with approximately the same number deemed to be undiagnosed due to lack of symptoms in approximately half of the affected population. Because of the severity of the disease endpoints (i.e. disability, limb amputation, death), easier, more accessible tools will help identify patients with PAD and diabetes at earlier stages of the disease by primary care physicians, enabling earlier intervention and avoidance of many of the disease's more severe outcomes.
PAD puts patients at elevated risk for lower extremity atherosclerosis, as well as for CHD or CVD, heart attack, stroke, and amputation. Approximately 75% of patients having PAD also have CHD or CVD. Risk of stroke is three times higher in patients with PAD than in those without the condition. PAD manifests as stenosis or obstruction of the arteries in the lower extremities and is caused by several factors including atherosclerosis, thrombosis, arterial calcification, diabetes, homocysteinemia, etc. Characterized by calf pain and disability, specifically claudication, and restricted ambulation due to critical limb ischemia, PAD is a progressive chronic disease—however, it should be noted that approximately half of all patients with PAD were free of symptoms at the time of their diagnoses.
Current diagnostic methods are typically applied to patients who present with symptoms of claudicating or leg pain at rest. A common diagnostic pathway includes use of the Ankle-Brachial Index (ABI) either at rest or during exercise, reactive hyperemia, photoplethysmography, segmental blood pressure analysis, pulse volume recording, duplex ultrasound, and peripheral angiography.
The ABI is typically the first test deployed and is usually performed in a physician's office or hospital vascular laboratory. The ABI is calculated from observations of systolic blood pressures taken from the brachial artery and at the ankle using sphygmomanometers and Doppler ultrasound. Although the ABI is considered the gold standard for non-invasive diagnosis of PAD, it is time-consuming and awkward to deploy, it is subjective, and it is technique-dependent. Thus, a relatively high and specialized training and experience level of the practitioner is required in order for consistent, reliable results to be obtained. Further, the ABI is not useful in the presence of arterial calcification, commonly encountered in patients at risk for PAD. This is because ABI relies on non-invasive blood pressure (NIBP) measurements that are confounded by arterial calcification.
Conventional photoplethysmography devices measure the volume of blood in a region of a subject's tissue. Conventional pulse oximeters measure how much oxygen binds to hemoglobin in red blood cells in a region of a subject's tissue. Neither concerns itself with a measure of quasi-periodic or cardio-rhythmic blood flow or circulation in a subject's extremity.